The present invention relates to improved hot-dip galvanized (zinc or zinc alloy) steel parts or sheets obtained by including a metal phosphide pigment, and preferably a ferrophosphorus pigment, in the hot-dip galvanized layer. The welding improvements realized by practicing the present invention are improved weldability lobes and dynamic resistance curves for better welding control for resistance welding systems, as well as increased electrode life and improved appearance and paintability.
The use of galvanized steel sheets in the automotive industry has become increasingly popular in recent years due to the increase in concern for corrosion protection for automobile body panels. Corrosion problems are particularly severe in environments where salt is used for preventing the icing of snow on highway roads. Although efforts have been made to enhance the corrosion-resistance of steel sheets, such as by using various chemical conversion treatments and paint coatings, the corrosion protection method of choice currently is galvanized steel, with the galvanized coating formed by either hot-dipping or electrodeposition.
Hot-dip galvanized coatings are applied by dipping or immersing the steel sheet or part in molten zinc to produce a coating having a thickness typically of about 0.003 to 0.15 mm. In a typical modern industrial process, the steel surface is preoxidized at 650.degree. C. and then hydrogen-reduced at 850.degree. C. to 950.degree. C. The temperature is lowered to 400.degree. C. with the strip still protected in hydrogen until it enters the zinc bath. In this way, flux at the entrance to the bath is avoided and small amounts of aluminum are used to inhibit formation of zinc-iron intermetallic intermediate layers. The bath temperature is maintained at 450.degree.-460.degree. C. by the sensible heat of the incoming strip.
The strip can then be jet-finished at line speeds of up to 185 m/min. During this process, the strip rises vertically out of the zinc bath, carrying an entrained viscous layer of molten zinc. A row of horizontal jets of air impinge perpendicularly on one side of the strip, and cause a return flow of liquid metal into the bath. Sensors above the row of air jets meter the thickness of the coating and adjust the velocity of air flow by electronic feedback circuits so that the desired thickness on each side can be maintained continuously throughout the run.
The characteristic spangle of galvanized steel sheets results from the rate of crystallization of the molten zinc, which depends on the condition of the starting steel and the presence of minor additions to the melt. The latter lower the melting point of the zinc and, thereby, lower the cooling rate of the molten layer. Paint adherence on a galvanized sheet depends on the orientation of the zinc crystals in the spangle, but, in general, adherence tends to be not as good as on bare steel. The spangle can be controlled by blowing zinc dust on the molten zinc surface and producing multi-nucleation sites for crystal formation to provide a highly nucleated, satin finish. However, this does not improve the poor resistance weldability of such coatings due to brass formation as discussed below.
Resistance spot welding is used to form joints between two materials. The process uses a set of electrodes to apply pressure to the weld area, to maintain the components in position, and to pass current through the weld. As the current flows, joule heating of the substrate occurs. Due in part to the cooling effects of the electrodes, a molten nugget eventually develops at the weld centerline or faying surface but not at the electrode contact. On cooling, this nugget resolidifies and effects a joining between the two materials. Some of the problems encountered during spot welding of galvanized steel sheets or parts include reduced weldability lobe widths, the absence of a dynamic resistance "beta peak", and decreased electrode life.
When resistance welding uncoated or bare steels, a single set of copper welding electrodes can be expected to make approximately 50,000 welds. When spot welding electrogalvanized steels, however, the zinc forms alloys with the copper electrode tip, forming a brass in situ. The brass sticks to the weld, rapidly eroding the welding tip which must then be replaced or refinished. This, in turn, reduces electrode life to about 1000-2000 welds or less. Since the production line must be stopped each time an electrode is replaced, at a considerable expense to the user, the relatively limited electrode life experienced when welding galvanized steels represents a significant economic disadvantage.
The use of ferrophosphorus pigment for both improved corrosion protection and weldability has been suggested in the prior art. For instance, U.S. Pat. No. 3,884,705, issued May 20, 1975, and U.S. Pat. No. 4,119,763, issued Oct. 10, 1978, both disclose the use of coatings containing ferrophosphorus and zinc pigments, and a non-metallic corrosion inhibitor such as zinc chromate, as a replacement for zinc-rich coatings. As contemplated in these patents, the ferrophosphorus pigment-containing coating is applied to bare steel panels rather than to galvanized sheets. The ferrophosphorus pigment used in such applications is commercially available from the Occidental Chemical Corporation under the trademark Ferrophos.RTM. pigment.
A ferrophosphorus pigment dispersed in a resin to bind adjacent steel plates to form a vibration-damping composite suitable for resistance welding is disclosed in Japanese Patent Application No. 61-41540, published on Feb. 27, 1986.
The use of a coating comprising a resin, ferrophosphorus powder and mica powder applied to a steel sheet having a layer of fused aluminum or an aluminum/zinc alloy is disclosed in Japanese Patent Application No. 591456884, published Aug. 22, 1984. The steel sheet described in this reference can be subjected to chemical conversion, and is further described as having excellent weldability, processability and corrosion and heat resistance.
The use of an iron layer containing less than about 0.5 weight percent phosphorus applied to a zinc/iron or zinc/nickel alloy electroplated steel sheet for improved surface properties is described by Honjo et al. in Internal Journal of Materials and Product Technology, Vol. 1, No. 1, pp. 83-114 (1986).
It will be appreciated by those skilled in the art that a continuing need exists for steel sheets or parts which posess the durability and corrosion resistance of galvanized components, but also possess the weldability advantages of bare steel. It would also be desirable to modify an existing coating line in order to accomplish this objective with minimal expense.